There is a long term and continuing effort to develop more effective seals for valves used to control the flow of corrosive fluids under high pressure. One of the more commonly used types of valves for this purpose is a plug valve. A plug valve has a fluid control member in the form of a plug which is rotatably fitted within the valve body. The plug of the valve is joined by an actuating shaft, which actuating shaft usually extends out of an access opening in the valve body to interact with an actuator. The actuator transmits torque through the actuating shaft to rotate the plug within the valve body and to selectively bring a passageway in the plug portion into and out of registry with a flow passage extending through the valve body. A valve cover having an opening for the actuating shaft is fitted over the access opening.
High performance plug valves used for high pressure corrosive fluids typically employ apertured sealing sleeves about the plug members. These apertured sleeves, which are apertured to permit fluid flow through the flow passage in the valve body, provide the primary seal against fluid leakage through the access opening. However, since some leakage past these sleeves sometimes occurs, it is common practice to provide additional seals, typically diaphragm seals, about the access opening and actuating shaft.
A number of different types of diaphragm seals have been used in the past to seal the area about the actuating shaft and the access opening. In one approach, a wedge ring, sometimes referred to as a delta ring, circumscribes the actuating shaft with an interference fit. A diaphragm with a central opening for the actuating shaft is then fitted about the stem in overlaying relationship to the delta ring. This type of arrangement is illustrated in U.S. Pat. No. 3,703,910.
Another successful access opening seal uses a diaphragm with a reverse lip which engages and extends along the actuating shaft's periphery. In this sealing arrangement, the opening in the diaphragm for the actuating shaft is in substantially perpendicular relationship to the outer and intermediate portions of the diaphragm.
Still another highly successful type of arrangement for sealing the actuating shaft opening of a rotary valve is disclosed in U.S. Pat. No. 4,333,632. In this sealing arrangement, a plug valve with an actuating shaft extending through the access opening has a sealing arrangement that includes a delta ring and the lip of a diaphragm. The diaphragm is formed with a hollow groove proximal to the actuating shaft, and the delta ring is placed in the hollow groove with an interference fit. The plastic diaphragm, usually formed from a fluorinated hydrocarbon material, is backed at its intermediate and radial outermost portions by a metal diaphragm which is partially fitted over the plastic diaphragm. The metal backup diaphragm functions to prevent excessive cold flow of the plastic diaphragm.
Due to the many difficulties of sealing metal surfaces against fluid leakage, including, adjoining, the non-pliant nature of metal and the varying dimensional changes that result from thermal expansion of various metal parts, such as relatively thin metal diaphragms and relatively massive valve actuating shafts, virtually all sealing arrangements for actuating shafts in valves designed for use under conditions of thermal cycling contact the actuating shaft with non-metallic materials. Indeed, all of the sealing surfaces that contact in the sealing arrangements discussed above are formed of plastic material, usually a fluorinated hydrocarbon polymeric material, such as polytetrafluoroethylene. Since such plastic materials disintegrate under the extreme heat conditions of a fire, alternative or supplemental fire-tested sealing arrangements must be used when the valve is used to control the flow of many highly flammable fluids. One such typical and commonly used supplemental sealing arrangement used to provide a supplemental fire-tested seal for the plastic diaphragm seals of the type discussed above is a flexible graphite stem seal cartridge. In this supplemental sealing arrangement, a flexible graphite sealing material is interposed between inner and outer thrust collars, and positioned in contacting relationship with the surface of the actuating shaft. These thrust collars are fitted onto a stamped metal diaphragm used to back up the radial outermost and intermediate portions of the plastic diaphragm. Thus, the metal diaphragm backs up the intermediate and radial outermost portions of the plastic diaphragm, and the thrust collars provide axial pressure back up for the circumferential area about the actuating shaft. The flexible graphite material is held in contacting relationship with the actuating shaft by the thrust collars, and the flexible graphite material is used to maintain a fluid seal against the actuating shaft in the event the plastic material used to seal about the actuating shaft is burnt away.
Due in part to the pliant nature of the flexible graphite material that is placed in contact with the actuating shaft, flexible graphite sealing cartridges have been proved to be successful supplemental fire-tested seals. Unfortunately, such cartridges add considerable complexity and cost to the sealing arrangement.
The non-metallic materials used to contact and seal against the actuating shaft are usually electrical insulators. Thus, it is necessary to use a static eliminator, a metal ring providing an electrical path between the actuating shaft and under body in order to avoid static buildup between these two parts of the valve.